Rename DSharedHistogram to DRealHistogram
Rename DSharedHistogram to DRealHistogram, part2
Break out the boolean response-variable case

The one-vs-all classifier strategy makes K boolean predictors; boolean
predictors can take shortcuts - the responses are only ever 0 or 1, and
the squares of 0 or 1 are also 0 or 1 respectively.  Means the
sum-of-squares is also the sum, and a bunch of math simplifies.
Also move the AtomicXXXArray updaters into the Utils directory.
Have DRealHistogram and DBinomHistogram share DHistogram, which carries
the bulk of the code.  Got lots of virtual calls (might have to be
hand-inlined to dodge the v-call costs).  But uses a bunch less memory,
and can use a bunch-less-more with more work.
Try removing per-bin min/max gather

Bring an optflags thru DRF, so I can turn opts on and off.
Remove a now-tested untested.
DRFTest allows control over binning also.
Try turning off per-col min/max gathering.  Removes a major array
allocation.
Switch to a single min/max per histogram

(instead of per-bin).  Drop the debugging optflags.  More generous
comments on why the fine-grained split logic.  Tweak bin defaults.
Cleanup test
Merge w master